1. Field of the Invention
The present invention relates generally to flame retardant optical fiber buffer coating compositions, and more particularly, to flame retardant fiber optic buffer coating compositions that can be applied to coated optical fiber and cured at high speed. The invention also relates to optical fiber flame retardant coating compositions which can be cured more efficiently than commercial thermoplastic buffer compositions. The invention also relates to an optical fiber coated with flame retardant buffer coating composition, and to methods of making such optical fiber.
2. Description of Related Art
Optical glass fibers are frequently coated with two or more superposed radiation-curable coatings which together form a primary coating immediately after the glass fiber is produced by drawing in a furnace. The coating which directly contacts the optical glass fiber is called the “inner primary coating” and the overlaying coating is called the “outer primary coating.” In older references, the inner primary coating was often called simply the “primary coating” and the outer primary coating was called a “secondary coating,” but for reasons of clarity, that terminology has been abandoned by the industry in recent years. Inner primary coatings are softer than outer primary coatings.
Single-layered coatings (“single coatings”) can also be used to coat optical fibers. Single coatings generally have properties (e.g., hardness) which are intermediate to the properties of the softer inner primary and harder outer primary coatings. The relatively soft inner primary coating provides resistance to microbending which results in attenuation of the signal transmission capability of the coated optical fiber and is, therefore, undesirable. The harder outer primary coating provides resistance to handling forces such as those encountered when the coated fiber is ribboned and/or cabled.
Optical fiber coating compositions, whether they are inner primary coatings, outer primary coatings, or single coatings, generally comprise, before cure, a polyethylenically-unsaturated monomer or oligomer dissolved or dispersed in a liquid ethylenically-unsaturated medium and a photoinitiator. The coating composition is typically applied to the optical fiber in liquid form and then exposed to actinic radiation to affect cure.
Optical fiber comprising a waveguide, an inner primary coating and an outer primary (or secondary) coating typically has a diameter of approximately 250 microns. The inner primary coating typically has an applied thickness of 20-40 microns and the outer primary coating typically has an applied thickness of about 20-40 microns.
For the purpose of multi-channel transmission, optical fiber assemblies containing a plurality of coated optical fibers have been used. Examples of optical fiber assemblies include ribbon assemblies and cables. A typical ribbon assembly is made by bonding together a plurality of parallel oriented, individually coated optical fibers with a matrix material. The matrix material has the function of holding the individual optical fibers in alignment and protecting the fibers during handling and installation. Often, the fibers are arranged in “tape-like” ribbon structures, having a generally flat, strand-like structure containing generally from about 2 to 24 fibers. Depending upon the application, a plurality of ribbon assemblies can be combined into a cable which has from several up to about 1000 individually coated optical fibers. An example of a ribbon assembly is described in published European patent application No. 194891. A plurality of ribbon assemblies may be combined together in a cable as disclosed, for example, in U.S. Pat. No. 4,906,067.
The term “ribbon assembly” includes not only the tape-like ribbon assembly described above, but optical fiber bundles as well. Optical fiber bundles can be, for example, a substantially circular array having at least one central fiber surrounded by a plurality of other optical fibers. Alternatively, the bundle may have other cross-sectional shapes such as square, trapezoid, and the like. Coated optical fibers (or waveguides) whether glass, or, as has come into use more recently, plastic, for use in optical fiber assemblies are usually colored to facilitate identification of the individual coated optical fibers. Typically, optical fibers are coated with an outer colored layer, called an ink coating, or alternatively a colorant is added to the outer primary coating to impart the desired color.
The ink layer, if applied, typically has an applied thickness of about 4-8 microns. The optical fiber, coated with inner primary coating, outer primary coating, and ink layer typically has a diameter of about 260 microns.
Typically, the matrix material of a fiber optic ribbon assembly or cable is separated from the individual coated fibers in order to facilitate splicing two cables, or the connection of a fiber to an input or output. It is highly desirable that the matrix material can be removed from the coated fiber with little or no effect on the outer primary coating or colored ink coating of the fiber. Good removability of the matrix material not only preserves the readily visual identification of the color-coded fiber, it also avoids harming the waveguide during the removal process.
It is well known in the art that optical fiber coated with well-known inner primary, outer primary, and ink or colored coatings have a relatively small diameter that makes such fiber difficult to work with and not entirely satisfactory for handling purposes. It is known to bundle optical fiber in loose buffer tubes. Such tubes include optical fiber surrounded by a gel-type buffer layer which is surrounded by the tube material. In order to improve handleability, and to add to the protection of the optical fiber, it is known to “upjacket” the fiber with a tight buffer coating. Upjacketing of the optical fiber is typically carried out to increase the diameter of the fiber of from about 250 microns to a diameter of from about 600 microns to about 900 microns. In a preferred form, the increased diameter of the fiber falls within the range from about 400 microns to about 900 microns. Upjacketing is desirable for applications such as local area networks, in-home applications, and in commercial establishments. Upjacketed fiber can be bundled without the need for additional gel filling or buffering in loose buffer tubes known in the art.
Because the optical adhesive and durability properties of the tight-buffer coating are not as rigid as those properties are for the inner primary, outer primary, and ink compositions typically used to make optical fiber, extruded thermoplastic materials such as polyvinyl chloride have been used heretofore as the tight-buffer coating. However, thermoplastic materials, such as polyvinyl chloride-based tight-buffer coatings are undesirable, particularly as the demand for tight-buffer coated optical fiber rises.
Equipment for applying extruded thermoplastic buffer coatings is expensive, thermoplastic materials are not suitable for short runs, and it is difficult to apply such coatings. Other drawbacks of thermoplastic coatings are that they must be heated during application, they must be extruded through relatively small dies, e.g., on the order of 250 microns to 900 microns, they must be cooled which can result in undesired stresses in the optical fiber and they are not adapted to be applied at the high line speeds at which optical fiber is made. Various attempts have been made to apply extruded thermoplastics to coated optical fiber at high line speeds, such as at speeds in excess of 100 meters/minute. Application of extruded thermoplastics at such line speeds has been unsatisfactory because the thermoplastic buffer coatings are not readily strippable from the optical fiber. Stripping the thermoplastic buffer coating has been found to cause damage to the underlying layers of ink, secondary or primary coatings. It is also known that attempts to apply extruded thermoplastics at high line speeds can result in unacceptable microbending induced signal-loss attenuation.
Recently, the art has attempted to provide a UV light-curable tight-buffer coating. For example, U.S. Pat. No. 6,208,790 B1 describes such-a coating, but this patent does not describe flame-retardant tight-buffer coatings, and it does not describe UV light-curable coatings which are flame retardant.
It would be advantageous in the art to provide a flame retardant tight-buffer coating composition, suitable for upjacketing optical fiber, that is curable by exposure to actinic, i.e., ultraviolet, radiation as well as such a coating that can be used on existing machinery and in existing processes well known to producers of optical fiber. Such machinery includes but is not limited to the machinery for applying ink to coated fiber and to ribbon-making machinery. Additionally, it would be desirable if the flame retardant tight-buffer coating is easily removed from the fiber without damage to underlying ink, secondary or primary coatings. It would be especially desirable if the flame retardant tight-buffer coating could be applied to coated optical fiber and cured at high speeds without causing unacceptable microbending induced signal-loss attenuation to the optical fiber. Thus, there remains a need for a UV-curable flame retardant buffer material that can be applied and cured at high speed, without causing unacceptable microbending signal-loss attenuation. There is also a need for a UV-curable flame retardant buffer material that is easily removed from the optical fiber without causing damage to underlying ink, secondary and/or primary coating layers. In its preferred embodiment, the present invention provides a composition that has these and, optionally, other desirable attributes as well.